1. Field of the Invention
The invention relates to a synthetic test circuit for a metal-encapsulated high-voltage power circuit breaker of a type which is grounded on one side and which includes several switching gaps arranged in series. More particularly, the invention relates to a synthetic test circuit comprising a high-current source and a high-voltage source for stressing the switching gaps which are operated during the test.
2. Description of the Prior Art
In metal-encapsulated high-voltage switching installations for 123 and 245 kV, synthetic test circuits are available which allow testing all the switching gaps which are arranged in series in the circuit breaker. In this connection, what is meant by synthetic is a type of testing in which a relatively low voltage, high current source for passing current through a circuit breaker to be tested and a relatively low power high voltage source for supplying an additional voltage across the terminals of the breaker, when it is open, to simulate the recovery voltage which exists under full voltage, full power conditions are provided. The use of such a test circuit, is based on the recognition that a switch to be tested must be loaded with high current with a closed contact system in which case the magnitude of the voltage is of secondary importance, i.e., it may be comparatively lower. Only when the switch is opened must the returning voltage appear at its contacts. The high load has already ended at this point. This gives rise to the term "synthetic" which means that there is a time staggered effect of current load and voltage load in the circuit breaker to be tested. This type of testing arrangement is disclosed in U.S. Pat. Nos. 3,064,183, and 2,898,548. In U.S. Pat. No. 3,064,183, a system is disclosed in which, as shown on FIG. 1 thereof, in order to test a circuit breaker there is provided a high voltage source 14 comprising a capacitor with an inductance in series. In series with the inductance and capacitor is a spark gap which is controlled by contol apparatus fed by a current transformer. In operation, when the spark gap breaks down, a voltage having the desired characteristics is simulated and applied across the open contacts of the breaker. The system also includes a high current source, feeding a transformer through a switch and a current limiting coil. The secondary of the transformer applies the high current to the circuit breaker in the closed condition. For installations operating at higher voltages of for example, 550kV or more, a test of all switching gaps arranged in series would require considerable expense for building the synthetic test circuit. This expense can be reduced if, instead of all the series-connected switching gaps, only one or some are acted upon by the high-current source and the high-voltage source of the synthetic test circuit. However, in such case, the high-voltage circuit breaker, which is grounded on one side, is tested at only a portion of its rated voltage. While this is of no importance for the testing of the individual switching gaps, it is significant for the testing of the insulation between the switching gaps and the metal encapsulation. More particularly, the test results give no information of the condition of the insulating medium between the open switching gap and the encapsulation because the voltage stress applied is only a portion of the rated voltage of the breaker.
It is, therefore, an object of the present invention to provide a synthetic test circuit which does not suffer the aforesaid disadvantage.